Personal care absorbent products, such as infant diapers, adult incontinent pads, and feminine care products, typically contain an absorbent core that includes superabsorbent in a fibrous matrix. Superabsorbents are water-swellable, generally water-insoluble absorbent materials having a liquid absorbent capacity of at least about 10, preferably of about 20, and often up to about 100 times their weight in water. While the core's liquid retention or storage capacity is due in large part to the superabsorbent, the core's fibrous matrix provides the essential functions of liquid wicking, pad strength and integrity, and some amount of absorbency under load. These desirable properties are attributable to the fact that the matrix includes cellulosic fibers, typically wood pulp fluff in fiber form.
For personal care absorbent products, U.S. southern pine fluff pulp is used almost exclusively and is recognized worldwide as the preferred fiber for absorbent products. The preference is based on the fluff pulp's advantageous high fiber length (about 2.8 mm) and its relative ease of processing from a wetlaid pulp sheet to an airlaid web. However, these fluff pulp fibers can absorb only about 2-3 g/g of liquid (e.g., water or bodily fluids) within the fibers' cell walls. Most of the fibers' liquid holding capacity resides in the interstices between fibers. For this reason, a fibrous matrix readily releases acquired liquid on application of pressure. The tendency to release acquired liquid can result in significant skin wetness during use of an absorbent product that includes a core formed exclusively from cellulosic fibers. Such products also tend to leak acquired liquid because liquid is not effectively retained in such a fibrous absorbent core.
The inclusion of absorbent materials in a fibrous matrix and their incorporation into personal care products is known. The incorporation of superabsorbent materials into these products has had the effect of reducing the products' overall bulk while at the same time increasing its liquid absorbent capacity and enhancing skin dryness for the products' wearers.
A variety of materials have been described for use as absorbent materials in personal care products. Included among these materials are natural-based materials such as agar, pectin, gums, carboxyalkyl starch and carboxyalkyl cellulosic, such as carboxymethyl cellulose. Natural-based materials tend to form gels rather than maintaining a solid form and are therefore not favored in these products. Synthetic materials such as polyacrylates, polyacrylamides, and hydrolyzed polyacrylonitriles have also been used as absorbent materials in personal care products. Although natural-based absorbing materials are well known, these materials have not gained wide usage in personal care products because of their relatively inferior absorbent properties compared to synthetic absorbent materials such as polyacrylates. The relatively high cost of these materials has also precluded their use in consumer absorbent products. Furthermore, many natural-based materials tend to form soft, gelatinous masses when swollen with a liquid. The presence of such gelatinous masses in a product's core tends to limit liquid transport and distribution within the core and prevents subsequent liquid insults from being efficiently and effectively absorbed by the product.
In contrast to the natural-based absorbents, synthetic absorbent materials are generally capable of absorbing large quantities of liquid while maintaining a relatively non-gelatinous form. Synthetic absorbent materials, often referred to as superabsorbent polymers (SAP), have been incorporated into absorbent articles to provide higher absorbency under pressure and higher absorbency per gram of absorbent material. Superabsorbent polymers are generally supplied as particles having a diameter in the range from about 20-800 microns. Due to their high absorbent capacity under load, absorbent products that include superabsorbent polymer particles provide the benefit of skin dryness. Because superabsorbent polymer particles absorb about 30 times their weight in liquid under load, these particles provide the further significant advantages of thinness and wearer comfort. In addition, superabsorbent polymer particles are about half the cost per gram of liquid absorbed under load compared to fluff pulp fibers. For these reasons it is not surprising that there is a growing trend toward higher superabsorbent particle levels and reduced levels of fluff pulp in consumer absorbent products. In fact, some infant diapers include 60 to 70 percent by weight superabsorbent polymer in their liquid storage core. From a cost perspective, a storage core made from 100 percent superabsorbent particles is desirable. However, as noted above, such a core would fail to function satisfactorily due to the absence of any significant liquid wicking and distribution of acquired liquid throughout the core. Furthermore, such a core would also lack strength to retain its wet and/or dry structure, shape, and integrity.
Another drawback of synthetic superabsorbent polymers is their lack of ability to biodegrade. The synthetic polymers' non-biodegradability is disadvantageous with regard to the disposal of used absorbent products containing these polymers.
Cellulosic fibers provide absorbent products with critical functionality that has, to date, not been duplicated by particulate superabsorbent polymers. Superabsorbent materials have been introduced in synthetic fiber form seeking to provide a material having the functionality of both fiber and superabsorbent polymer particle. However, these superabsorbent fibers are difficult to process compared to fluff pulp fibers and do not blend well with fluff pulp fibers. Furthermore, synthetic superabsorbent fibers are significantly more expensive than superabsorbent polymer particles and, as a result, have not competed effectively for high volume use in personal care absorbent products.
Cellulosic fibers have also been rendered highly absorptive by chemical modification to include ionic groups such as carboxylic acid, sulfonic acid, and quaternary ammonium groups that impart water swellability to the fiber. Although some of these modified cellulosic materials are soluble in water, some are water-insoluble. However, none of these highly absorptive modified cellulosic materials possess the structure of a pulp fiber, rather, these modified cellulosic materials are typically granular or have a regenerated fibril form.
A need exists for a highly absorbent material suitable for use in personal care absorbent products, the absorbent material having absorptive properties similar to synthetic, highly absorptive materials and at the same time offering the advantages of liquid wicking and distribution associated with fluff pulp fibers. Accordingly, there is a need for a fibrous superabsorbent that combines the advantageous liquid storage capacity of superabsorbent polymers and the advantageous liquid wicking of fluff pulp fibers. Ideally, the fibrous superabsorbent is economically viable for use in personal care absorbent products and is biodegradable thereby making the disposal of used absorbent products environmentally friendly. The present invention seeks to fulfill these needs and provides further related advantages.